Currently most of the research and applications are concerned with composites whose matrix is made of polymers, ceramic materials or metals (e. g. aluminium, titanium, nickel and other alloys) and which are reinforced with glass or carbon fibres. Such composites have found wide application in the manufacturing of fuselages and other structural components of planes, gliders, space shuttles and rockets, car body components, fuel tanks, yacht hulls and masts, wind turbine blades and domestic appliances components, in medicine and in many other fields. Moreover, composites are used to reinforce the existing building structures, including the historic ones (e. g. to preserve bridges, churches, etc., or to increase their load-bearing capacity).
The percentage of composite materials used in high-tech applications increases every year, causing a rise in the production of reinforcing fibres. It is estimated that the demand for carbon fibres in 2018 will increase by about 275% (from 41240 to 113500 tons) as compared with that in 2010, (Sloan, 2010). The highest increase in demand is forecasted for applications connected with the wind power industry (700%), the storage of compressed fuels (~410%) and the broadly understood aerospace industry (~220%).
The interest in composite materials is due to their very good mechanical (strength) parameters and low specific gravity. Highly valuable is the possibility of creating a composite with strictly defined parameters for a given structure. Moreover, increasingly often various (mainly optical fibre based) sensors are incorporated into the structure of composite materials. Thanks to this, different parameters (e. g. strain and temperature) can be measured directly in the structure’s material. Another major advantage of optical fibre sensors (OFS) is that they can be used to detect defects and their accumulations and to determine the safe life of the material during its service. In recent years most of the research
on OFS has been devoted to their use for the continuous monitoring of the structural health of composite elements.
Novel structural health monitoring (SHM) systems for composite structures are currently developed. Thanks to the use of measuring systems integrated with the structure the latter can be optimally managed. Continuous monitoring enables the detection of hazards in the structure, undetected by inspections or modelling. Consequently, the damaged components can be promptly replaced whereby the safe service life can be extended. Moreover, thanks to SHM systems the knowledge about structures and their behaviour in real conditions can be improved whereby the next generation of such structures will be better designed. Ultimately, the costs connected with building new structures will be lower and the safety, reliability and life of the latter will increase (Glisic & Inaudi, 2007).